For some years, we have been using the unique tool of inbred histocompatible guinea pigs for investigation of mechanisms of hypersensitivity in the pathogenesis of and the protection against infectious and autoimmune diseases. Advantages lie in the ability to perform viable adoptive transfer of lymphoid cells, and in the capacity to study genetic factors in certain hypersensitivity and autoimmune phenomena. We have concentrated recently on the pathogenesis of an age-dependent induction of cataracts in immature guinea pigs undergoing the paralysis and atrophy of autoimmune encephalomyelitis and on the prevention and treatment of this condition. The cataracts are reminiscent of some clinical or experimental entities shown to be due to nutritional deficiency or enzymatic alteration and may be secondary to the autoimmune reaction in the host.